As is well known, demands for a holographic digital data storage system that can store a large amount of data have been increasing. Therefore, various types of holographic digital data storage systems have been recently developed for realizing high density optical storage capabilities.
The holographic digital data storage system, e.g., a holographic ROM system, allows a modulated signal beam carrying digital data (to be stored) to interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored in a storage medium, e.g., a disk-shaped holographic medium, made of, e.g., an optical refractive crystal. The optical refractive crystal is a material which may react differently on different amplitudes and phases of the interference pattern.
In the holographic digital data storage system, various holograms can be recorded in a same spatial location by changing the angle of incidence of the reference beam (angular multiplexing), so that holograms can be stored in the holographic medium. After the holograms are recorded in the holographic medium, the disk-shaped holographic medium is inserted into, e.g., a holographic ROM player for replaying the holograms recorded in the holographic medium.
During playback, only a reference beam, which is preferably identical to that employed during the recording operation, is irradiated onto the holographic medium. The reference beam is then diffracted by the interference pattern recorded in the holographic medium to thereby reproduce the recorded binary data.
FIG. 1 shows a conventional holographic ROM system during playback (see “Holographic ROM system for high-speed replication”, ISOM/ODS 2002, pp 144˜146). The conventional holographic ROM system includes a light source 1; a beam reducer 2; mirrors 3a, 3b; and a holographic medium 4.
The light source 1 emits a reference beam with a constant wavelength, e.g., a wavelength of 532 nm. The reference beam is then provided to the beam reducer 2 for reducing the beam size of the reference beam down to a predetermined size. The reduced reference beam is reflected by the mirror 3a toward the mirror 3b. Thereafter, the mirror 3b reflects the reference beam toward a location of the holographic medium 4. The mirror 3b can be, e.g., rotated for reflecting the reference beam toward another layer of the holographic medium 4. The reference beam is then diffracted by the interference pattern recorded in the holographic medium 4 to thereby produce a regenerated beam.
FIG. 2 presents a conventional pickup device for replaying data recorded on a plurality of tracks of the holographic medium 4 of FIG. 1 in a holographic ROM player. The pickup device includes lenses 5, 6, 8; a pinhole 7; and a detector 9. In case the reference beam is incident on a location of the holographic medium 4 as shown in FIGS. 1 and 2, the regenerated beam is emitted from the location of the holographic medium 4. Since the size of the regenerated beam is, e.g., 100 μm, and a track pitch, which represents a spacing between tracks on the holographic medium 4, is, e.g., 0.74 μm, the regenerated beam includes therein quantities of data which are read from hundreds of tracks on the holographic medium 4.
Then, the regenerated beam is incident on the lens 5 which collimates the regenerated beam. The collimated beam is provided to the lens 6, which focuses the collimated beam at the pinhole 7 to thereby produce a focused laser beam. The focused laser beam including the quantities of data read from the hundreds of tracks is sifted through the pinhole 7, thereby being converted into the sifted laser beam including only data read from a target track. Subsequently, the sifted laser beam including only the data read from the target track is provided to the lens 8, which focuses the sifted laser beam at the detector 9. The detector 9 detects the data read from the target track in order to replay the hologram recorded on the target track.
However, the plurality of tracks formed on the holographic medium 4 are in a spiral shape and the holographic medium 4 may wobble when the holographic medium 4 rotates during playback. Therefore, a pickup apparatus is required for controlling the tracking and the focusing of the laser beam on a target track of the holographic medium 4. However, such a pickup apparatus for reproducing data from the disk-shaped holographic medium is not yet available although pickup apparatuses for either CD players or DVD players have already been commercialized.
Therefore, there has existed a need for a pickup apparatus for data reproduction in a holographic storage system to control minute changes in the tracking or the focusing operation.